A sex alien is a common antagonist in science fiction stories. It is a creature with supernatural sexual powers. A sex alien can often induce a trance-like state of arousal in their victims through touch or kiss. A sex alien can often change appearance, and sometimes even gender, to better fit the desires of the intended victim. Similar to the succubus of medieval legend, intercourse with a sex alien will almost certainly result in death as the sex alien feeds on the sexual energy of its victim. When explaining a sex alien's powers, there is about a 75% chance the word, "pheromones" will be used. Any sc-fi television series to feature aliens will inevitably feature a sex alien. The only exception to this rule occurs when a show is cancelled before reaching that point.

The Kindred in the 13th episode of the first season of the X-Files, the Orion slavegirls from Star Trek, and the gaseous alien from the second episode of Torchwood are all examples of a sex alien.